In image-forming devices using electrophotography, such as a coping machine, a facsimile machine and a printer, an image is formed by at first developing an electrostatic latent image formed on a photosensitive member with a toner for developing electrostatic image, which may be referred to merely as a toner hereinafter, next transferring the formed toner image on a recording material such as a paper piece, and then fixing the image in various manners such as heating, pressing and solvent evaporation.
In recent years, for these image-forming devices, energy consumption has been required to be decreased and printing has been required to be made highly speedy. Out of steps of forming an image according to electrophotography, the step of consuming a particularly large quantity of energy is a step of fixing a toner on a recording material (fixing step). The manner used in the fixing is generally a manner of attaining the fixing by heating and pressing, using a fixing roller. In this manner, usually it is necessary to make the temperature of the fixing roller high in the fixing. From the viewpoint of energy conservation, this temperature of the fixing roller in the fixing is required to be made low. Furthermore when the fixing can be attained at a low temperature, the fixing can cope with printing at a high speed; therefore, the temperature of the fixing roller in the fixing is required to be made low also from the viewpoint of the requirement of making the printing speed high.
About toner, a toner wherein the minimum fixing temperature thereof is low is desired. In order to yield a toner excellent in low-temperature fixability, the following methods have been hitherto suggested: a method of lowering the glass transition temperature of a toner; a method of adding a resin having a low melting point or a low molecular weight, or the like; a method of incorporating, into a toner, a low-softening point substance having a releasing characteristic, such as wax; and other methods.
In the meantime, the shelf stability of toner is required as a characteristic conflicting with the low-temperature fixability. The shelf stability is a characteristic showing the degree of a matter that when a toner is stored at high temperature, aggregation (blocking) of the toner is not easily caused. This characteristic is a characteristic necessary for causing a toner not to aggregate when the toner is transported or when the temperature of the inside of a printer is raised by heat-fixing generated in printing.
In order to attain the above-mentioned required themes, suggested are techniques about resin or wax to be added to toner.
Patent Document 1 suggests a toner containing a 1-olefin polymer wax that is made of units each derived from a 1-olefin represented by R′CH═CH2 wherein R′: an alkyl having 1 to 28 carbon atoms, and that has a DSC melting point of 70 to 130° C., a molecular weight M of 100 to 25,000 and a molecular weight distribution Mw/Mn of 1.5 to 3.0 (claim 1), and discloses a toner containing a polypropylene wax having a DSC melting point of 81 to 120° C.
Patent Document 2 suggests a solid toner for wet development wherein toner particles comprising a colorant, a binder resin and so on (colored resin particles) are dispersed in an electrically insulating medium containing: a polymer wax obtained by polymerizing an α-olefin having 19 to 60 carbon atoms; and a crystalline wax.
However, it has been demonstrated by the investigations of the present inventor that when the toner using the wax described in Patent Document 1 is used in recent color printers, the low-temperature fixability is not yet sufficient against the requirement that the low-temperature fixability should be made better while the existing shelf stability is kept.
The polymer wax described in Patent Document 2, which is obtained by polymerizing the α-olefin, is a wax used as a medium for a solid toner for wet development (that is, a matrix in the toner). When the wax is used as a component contained in colored resin particles, there arises a problem that the shelf stability of the toner lowers.
Moreover, the printing durability of toner is also required as a characteristic conflicting with the low-temperature fixability. The printing durability is a characteristic that even when printing is made on a large number of sheets, printing qualities such as image quality can be maintained. In a toner cartridge, a toner is required to be less deteriorated by stress which the toner undergoes from printing on a large number of sheets, and by other factors. The printing durability is required particularly not to generate fog when continuous printing is made.
Furthermore, as the formation of color images has been spreading in recent years, the quality required for color toner used in the formation has been becoming higher. Thus, the characteristics conflicting with the low-temperature fixability have also been requested to be further improved. There has been particularly desired a toner good in printing durability even after the toner is put in a high temperature environment when transported, or when the toner is used in printing in a high temperature environment.
In order to solve the above-mentioned problems while the requirement about the low-temperature fixability is satisfied, various suggestions are made.
In order to yield toner excellent in both of low-temperature fixability and printing durability, Patent Document 3 discloses a toner, for developing electrostatic image, containing, as a parting agent, a paraffin wax having a melting point of 60 to 85° C. and a penetration of 4 to 10 at 25° C. However, when the parting agent described in Patent Document 3 is used, the printing durability deteriorates largely after the toner is stored at high temperature.
In order to yield toner excellent in both of low-temperature fixability and shelf stability, Patent Document 4 discloses a toner, for developing electrostatic image, containing 1 to 15 parts by weight of a wax about which an endothermic range based on DSC is present only in a temperature range of 50° C. or higher for 100 parts by weight of a binder resin. As the wax, a wax is used which is obtained by removing, from esters made from aliphatic acid and alcohol, fractions having a DSC endothermic range of 50° C. or lower, and purifying the resultant. However, when the wax described in Patent Document 4 is used, the printing durability deteriorates as well when the toner is stored at high temperature.
Patent Document 5 suggests a toner which contains an ester component having a penetration of 4 or less as a main essential component, and a wax having a melting point of 60 to 110° C., and which is produced by an emulsion polymerization coagulation method. However, the toner in Patent Document 5 is insufficient for compatibility at a high level between low-temperature fixability and shelf stability, and a level required about printing durability, which are associated with recent spread in color printers.
In order to yield toner excellent in all of low-temperature fixability, shelf stability, and printing durability, in particular printing durability when the toner is stored at high temperature, Patent Document 6 discloses a toner, for developing electrostatic image, comprising colored resin particles containing a binder resin, a colorant and a specific ester compound. The ester compound described in Patent Document 6 is characterized in that when the endothermic peak in the DSC curve thereof is 65° C. or higher, the endothermic value at a temperature 20° C. lower than the endothermic peak temperature is represented by A (mW), the endothermic value at a temperature 50° C. lower than the endothermic peak temperature is represented by B (mW), the number of ester bonds therein is represented by C and the amount of the ester compound used as the sample to be measured is represented by D (mg), a relationship represented by the following expression is satisfied:relational expression: 0≦|A−B|/(C×D)≦0.02
However, even when the ester compound described in Patent Document 6 is used, the printing durability after the toner is stored at high temperature becomes lower than that before the high-temperature storage.
A characteristic of toner required besides low-temperature fixability, shelf stability and printing durability is offset resistance. The offset resistance is a characteristic showing the latitude of a temperature range in which the toner can be fixed. In particular, about toner which can be fixed at low temperature, it is necessary that stable printing quality is obtained even when the temperature of a fixing roller is broad. Thus, the toner is required not to be adhered to the fixing roller even at a high fixing temperature.
Patent Document 7 discloses a toner containing an ester wax (corresponding to a “low molecular weight wax”) and a hydrocarbon wax (corresponding to an “aliphatic hydrocarbon polymer”) which each have a melting point in the range from 55 to 120° C., wherein the melting point of the ester wax is lower (claim 1). It is stated that the weight-average molecular weight (Mw) of the ester wax ranges from 350 to 1,500 and the weight-average molecular weight (Mw) of the hydrocarbon wax ranges from 300 to 4,000 (claims 3 and 4).
Patent Document 8 discloses a toner for developing electrostatic image containing a polyester as a binder resin, and an ester wax (corresponding to a “low molecular weight wax”) and a petroleum wax (corresponding to a portion of a paraffin wax “aliphatic hydrocarbon polymer”) as a parting agent (claims 1 and 2). Furthermore, it is stated that the melting point of the petroleum wax is preferably 70° C. or higher. The molecular weight of the ester wax and that of the petroleum wax are not described.
Patent Document 9 discloses a toner for electrophotography containing: a wax-containing resin obtained by polymerizing a monomer in the presence of a high melting point wax (polyethylene wax (corresponding to an “aliphatic hydrocarbon polymer”)) having a melting point of 75 to 140° C.; and a low melting point wax (ester wax (corresponding to a “low molecular weight wax”) having a melting point of 50 to 90° C. (claims 1 to 3). The molecular weight of the ester wax and that of the polyethylene wax are not described.
However, the toners of Patent Document 7 to 9 are insufficient for satisfying a recent high level requirement that a balance between low-temperature fixability and shelf stability is kept at a high level while toner should have offset resistance and printing durability.
Patent Document 1: Japanese Patent Application laid-Open (JP-A) No. 2000-352838
Patent Document 2: JP-A No. Hei. 9-106113
Patent Document 3: JP-A No. 2005-266753
Patent Document 4: JP-A No. Hei. 3-91764
Patent Document 5: JP-A No. 2000-35690
Patent Document 6: JP-A No. 2005-221571
Patent Document 7: JP-A No. 2004-251932
Patent Document 8: JP-A No. 2005-141189
Patent Document 9; JP-A No. 2003-5431